As a hybrid driving force transmission device, there has heretofore been known one in which an engine, a motor clutch unit and a transmission unit are coupled, as shown in Japanese Patent Application Publication No. 2006-137406. Among these, the motor clutch unit has a multi-plate dry clutch disposed inside an electric motor. Specifically, the motor clutch unit includes a clutch hub coupled to an output shaft of the engine, a clutch cover having a rotor of an electric motor fixed thereto, and coupled to a transmission input shaft, and a multi-plate dry clutch interposed between the clutch hub and the clutch cover.